I believe the theme in this story is "do not judge" or do not judge with prejudice. There is evidence that "do not judge" might be the theme of the book because of the quotes. For example, one quote states: "In order to know a person, you must get in their skin and crawl around in it", which was said by Atticus. That quote specifies that before you judge anyone, you must first see things from their perspective and see what they see, before you prejudge that person from what you think you know about them.